There has been a continuing need for a simple, effective, and relatively inexpensive lung assist device for supplying oxygen and removing carbon dioxide from patients suffering from acute illnesses such as pneumonitis, atelectasis, various heart and circulatory ailments, fluid in the lungs, obstruction of pulmonary ventilation, or lung injury caused by heat, noxious gases, or other factors. Prior techniques for so assisting the lungs fall into three general categories: respirators, extracorporeal oxygenators, and intravascular lung assist devices.
Respirators improve the efficiency of a patient's lungs but are less than suitable for use in those situations where a patient's damaged or diseased lungs require rest or are simply incapable of performing the work needed for adequate respiration. Extracorporeal oxygenators usually take the form of membrane oxygenators (ECMOs) commonly referred to as heart-lung machines and are most frequently used for relatively snort intervals, as during surgery where the circulation through a patient's heart and lungs is temporarily bypassed. Reports of successful longer-term usage of ECMOs in adults, except for particular categories of patients suffering from specific types of circulatory problems, have generally been lacking. Further, it is apparent that the use of ECMOs not only involves the deployment of expensive equipment but requires constant supervision and control by teams of skilled technicians. ECMO usage also necessitates the use of anticoagulants which may present problems of internal bleeding, especially when administered on a longer-term basis.
Intravascular lung assist devices (ILADs) are believed to overcome many of the shortcomings of respirators and extracorporeal oxygenators. U.S. Pat. No. 4,583,969 discloses a mass transfer device intended to be inserted into a patient's vena cava to provide extrapulmonary in vivo oxygenation of the blood. The device takes the form of a bundle of tiny gas-permeable tubes that extend between a pair of headers connected to gas (oxygen) supply means and to gas exhaust means, respectively. When located in the vena cava, the device operates as an intravascular artificial lung to assist the operation of a patient's diseased or damaged lungs. Reference may also be had to related U.S. Pat. Nos. 4,631,053 and 3,505,686 as further illustrating the state of the art.
Despite their advantages, prior ILADs have disadvantages that largely offset their usefulness. A chief shortcoming is their relatively poor gas-transfer efficiency. Another is size; if such a device is small enough to be inserted into the body through the femoral, iliac, or jugular veins, it does not efficiently utilize, for purposes of gas transfer, the lumen space at the ultimate location in the vena cava, and if the device were sized for more efficient performance at its operative site, it would be too large for introduction into the body through the branch veins.
Accordingly, a main aspect of this invention lies in providing an ILAD that overcomes or greatly reduces the problems associated with prior intravascular lung assist devices. In particular, this invention is concerned with an ILAD that is radially expandable in situ to provide relatively high gas transfer efficiency in use. Such efficiency is achieved not only because of the relatively large surface area exposed to blood flow, but also because of the cross flow arrangement of fibers that minimizes blood channeling around the device and the relatively short gas pathways and parallel flow circuitry involved.
In brief, the device takes the form of an elastomeric tubular core with longitudinal gas inflow and outflow passages. A plurality of bundles of flexible but relatively non-stretchable hollow fibers formed of gas-permeable polymeric material surround the elastomeric core, each of the bundles having end portions spaced from those of adjacent bundles and being fixed to the core by mounting collars and suitable embedding or potting means. Each of the bundles also has a flexible intermediate portion in which the fibers are substantially straight and parallel when the core is in stretched condition but in which the fibers flex outwardly to form a rosette of outwardly-bowed fibers when the axial stretching forces on the core are relieved. The core has lateral apertures disposed within the mounting collars for placing the end portions of successive hollow fiber bundles in flow communication with such passages. Such apertures are arranged to communicate alternately with the inflow and outflow passages within successive mounting collars along the length of the core. The bundles are therefore paired and arranged as subunits with parallel (rather than serial) flow of gases to and from the bundles of successive subunits.
The method of forming the ILAD of this invention involves the steps of first longitudinally stretching the elastomeric tubular core, then arranging a plurality of longitudinally-spaced bundles of hollow, flexible, gas-permeable fibers around the core, securing the end portions of each bundle to the core while the core is in its longitudinally stretched condition and the fibers are substantially straight, providing lateral openings in the core communicating with longitudinal inflow and outflow passages extending through the core, such openings alternately communicating with such inflow and outflow passages in the spaces between the end portions of successive bundles of fibers, and finally enclosing the spaces between successive bundles by collar elements that provide manifold chambers for the inflow and outflow of gases into and from the hollow fiber bundles. Thereafter, when the longitudinal stretching forces on the core are relieved, the core contracts axially and the fibers of each bundle flex outwardly to form a rosette of outwardly-bowed hollow fibers.
Since each manifold chamber between a pair of adjacent fiber bundles serves either as an inlet chamber or an outlet chamber, depending on the core passage with which it communicates, and since there is an alternating arrangement of inlet and outlet chambers along the bundle-providing length of the device, it is believed apparent that each manifold chamber serves two such bundles (except at the extreme ends of the series), that each pair of bundles constitutes an operating subunit, and that the gases flowing through the pair of bundles of each such subunit travel in opposite axial directions.
In a second embodiment of the invention, each bundle has one or more restraining rings extending about its intermediate portion. In such a construction, when the stretching forces on the elastomeric core are relieved, a rosette of outwardly-bowed fibers is formed on opposite sides of each such restraining ring.
In the use of the device, the elastomeric core is first stretched axially to cause the fibers of the bundles to straighten. Such longitudinal stretching may be achieved by any suitable means such as by inserting a relatively rigid member into at least one of the flow passages of the core. The device in its radially contracted condition is then inserted into the lumen of a blood vessel such as the femoral vein or artery. When the bundle-providing portion of the device has been advanced to its operative site in the vena cava or aorta, the core is permitted to retract longitudinally (as by withdrawing the rigid member from the core), thereby causing the fibers of the bundles to expand outwardly into a series of rosettes of outwardly-bowed fibers.
Other features, advantages, and objects of the invention will become apparent from the specification and drawings.